The present invention relates to a power on sequence for a non-volatile memory semiconductor device, and more particularly, to a power on sequence for a flash memory device that includes error bit check before reading data.
Non-volatile memory (“NVM”) refers to semiconductor memory which is able to continually store information even when the supply of electricity is removed from the device containing such an NVM. NVM includes Mask Read-Only Memory (Mask ROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM) and Electrically Erasable Programmable Read-Only Memory (EEPROM). Flash memory is also a type of NVM which may be considered an EEPROM.
Flash memory cells often use a floating-gate transistor including a source, a drain, a floating-gate layer and a control-gate layer. Access operations are carried out by applying biases to each of these respective terminals. Write operations are generally carried out by channel hot electron injection (CHE). The CHE process induces a flow of electrons between the source and the drain, and accelerates them toward a floating gate in response to a positive bias applied to the control gate. Erase operations are generally carried out through Fowler-Nordheim (FN) tunneling. The erase process may include electrically floating the drain, grounding the source and applying a high negative voltage to the control gate. Read operations generally include sensing a current between the source and the drain in response to a bias applied to the control gate. If the memory cell is programmed, the cell's threshold voltage will be near or above the control-gate bias such that the resulting current is low. If the memory cell is erased, the cell's threshold voltage is well below the control-gate bias such that the current is substantially higher. Other programming, erasing and reading techniques are known in the art.
FIG. 1 shows a basic flash memory device 100 for coupling to a processor or other controller U1. The flash memory device 100 includes a flash array 102, a row decoder 104 and a column decoder 106. The flash array 102 includes a plurality of rows and columns of memory cells accessible for reading, programming and erasing by the combination of the row and column decoders 104, 106. An address buffer 108 is coupled to an address pad 110 for receiving address information and applying the address information to the row and column decoders 104, 106. A sense amplifier 112 senses and amplifies data stored in the individual memory cells within the flash array 102. A data buffer 114 buffers the data received from the sense amplifier 112 and output to a data pad 116. A command control circuit 118 decodes information received from a control pad 120 and a power on reset (POR) pad 122. The command control circuit 118 controls data read, data write and erase operations in the flash memory device 100. Data read from the flash memory array can be selectively written to an information register 124 via a data bus 126.
There are more and more functions in devices containing flash memory which require auto loading of information stored by non-volatile cells in an information array 102a to the information register 124 during a power on sequence or power-up of the flash memory device 100. The non-volatile cells of the information array 102a are also within the flash memory device 100, and the information stored therein is typically important for some control functions. The data stored in the non-volatile cells of the information array 102a can be programmed, erased and read by using the same data path as the rest of a normal flash array 102.
FIG. 2 is a graph depicting device voltage versus time for a conventional power on sequence of a flash memory array 102. As shown, when the device voltage reaches the minimum value, e.g., 1.8 V, the power on reset (POR) stops and an immediate read of data from the memory array 102 may not be stable. The normal logic low value is higher than the minimum logic low value, e.g., approximately 3 V for normal versus 1.8 V for the minimum logic low value.
It is desirable to provide a power on sequence for a non-volatile memory semiconductor device. It is also desirable to provide power on sequence for a flash memory device that includes error bit check before reading data. It is desirable to ensure that a flash memory device has reached a stable voltage during a power on sequence by performing an error bit check before reading data stored in cells of the flash memory device.